Method and apparatus for viewing the impact spot of a charge carrier beam

ABSTRACT

In equipment using a beam of charged particles for machining a workpiece, the size and/or shape of the beam&#39;&#39;s impact spot is checked by placing a sharply defined edge to intercept energy radiated from the impact spot so that the radiation casts a shadow on a detection device, which is provided by a fluorescent screen of a photoelectric detector or by a combination of them.

maw

inventors Edgar Meyer Wessling;

Joachim Geisler, Munich, both of,

Germany Appl. No. 4,221 Filed Jan. 20, 1970 Division oi' Ser. No.804,342, Jan. 24, 1969, which is a continuation of Ser. No. 473,355,

July 20, 1965, abandoned.

Aug. 24, 1971 Steigerwald Strahlte'chnik GmbH Munich, Germany PatentedAssignee METHOD AND APPARATUS FOR VIEWING THE IMPACT SPOT OF A CHARGECARRIER BEAM l2 Claims, I2 Drawing Figs.

U.S. CI

Int. CI B231( 15/00 [50] FieldoiSearch 219/121, 121EB. 121 L, 69;Z50/49,5, 41.9

[56] References Cited I UNITED STATES PATENTS 2,901,627 8/1959 Wiskottet al 2SC/49.5

3,049,618 8/1962 Thome Z50/49.5

3,143,651 8/1964 Giacconi et al 250/105 3,196,246 7/1965 El-Kareh 219/69Primary Examiner-J. V. Truhe Assistant Examiner-Robert E. ONeillAiromey-Sandoe, Hopgood & Calimafde ABSTRACT: ln equipment using a beamof charged particles for machining a workpiece, the size and/or shape ofthe beams impact spot is checked by placing a sharply defined edge tointercept energy radiated from the impact spot so that the radiationcasts a shadow on a detection device, which is provided by a fluorescentscreen of a photoelectric detector or by a combination of them.

PAT-ENTEU 111112241911 SHEET l UF 5 F ig,2

invento#l FDGAR MEYFR JOACHIM GEISSLER PATENTEU Auzdlsm 3601,57?

snm 2 nr 5 Inventor:

EDGAR MEYER JOACHIM CEISSLER PATENEUAuGzMQn 3,601.57?

SHEET 3 nF 5 Fig.6

Hg] g g 7 Fig@ l 52 V12 jiji? lo I 5 f Inventor,

i f EbGAR MEYER I3 JOACHIM GEISSLER PATENT AUQQMSA SHEET I4 0F 5 Fig.9

Fig. n

Inventor:

EDGAR MEYER JOACHIM GEISSLER PATENTED M1924 um SHEU 5 UF 5 Fl`g.l2

ESSB/ Inventor,

EDGAR MEYER JOACHIM GEISSLER METHOD AND APPARATUS FOR VIEWING THE IMPACTSPOT F A CHARGE CARRIER BEAM This is a division of application Ser. No.804,342 filed Jan. 24, |969 as a continuation of application Ser. No.473,355, filed July 20, 1965, now abandoned.

This invention relates to an improved system for checking the focus of abeam of charged particles upon an impact spot in equipment using thebeam for machining of a workpiece.

Charge carrier beam equipment, and above all, equipment using such beamsfor the purpose of machining, require the beams impact spot on thesubject be observed in order that the focus can be adjusted to thedesired, usually minute size. In addition,`it is often desirable to beable to observe the shape of the beams impact spot, so that a certain,usually round focus, can be obtained by an appropriate adjustment ofvarious focusing fields.

It is of prior art to check the dimension of the beams focus in chargecarrier beam equipment by light-optical means. However, due to therelatively small aperture of the optical light path which normallypasses through the pole shoes of the electron-optical focusing lens,such a light-optical checking system has an insufficient resolvingpower. Small foci can, therefore, no longer be observed and checked withsufficient accuracy.

ln addition, most of the known types of light-optical viewing systemshave a glass lens'disposed in the corpuscular beam device proper, andthis lens is provided with a central bore to allow the corpuscular beamto pass. The optically best suited center portion of the lens is, thus,inevitably lost for observation, i.e., a degraded image has to betolerated. This is another reason whysmall foci cannot be observed withsufficient accuracy. i

Another drawback of the light-optical viewing system is the fact thatthe optical lens incorporated in the corpuscular beam device is quicklycovered with evaporated material in the course of the machining process,so that it becomes unsuitable for further use. A protective glass plateis, therefore, arranged in frontof the lens, which collects theevaporated material and must, therefore, be replaced from time to time.

lf a high-power beam is used, the protective glass must be replaced veryoften. This is, of course, accompanied by frequent, undesirableinterruptions of the work.

lt is also prior art to check the focusing of a beam of chargedparticles by arranging a collecting electrode above the workpiece to bemachined and to measure the current intercepted. When this currentreaches an extreme value, this is interpreted to be equivalent tooptimal focusing. This method has the disadvantage that it worksproperly only with a few materials and that, moreover, the adjustment isnot made for minimum diameter of focus, but for maximum temperature atany point of the impact area. ln addition, this method does not provideany information on the size and shape of the impact spot of the beam,

The purpose ofthe present invention is to indicate a process forchecking the size and/or shape of the beams impact spot, which is freefrom the disadvantages of previously known processes and which,moreover, offers a number of essential advantages. lt is also thepurpose of this invention to provide a device for carrying out the newprocess, which is of simple design in spite of its high efficiency.

The invention, thus, refers to a process for checking the size and/orshape of the beams impact spot in equipment using a charged particlebeam for machining. According to the invention, the particles and/orX-rays reflected from the impact spot of the beam are used for such acheck.

ln this connection, it is particularly advantageous to image the impactspot by charge carrier optical means, with the aid of the particleradiation reflected from it, onto a radiation detector which suppliesinformation on size'and shape of the image formed. lf, for example, afluorescent screen is used as a radiation detector, the correspondingsizes of the beams impact spot can be directly deduced from the size andshape of the image formed on this screen. In this manner, even small andminute impact spots can be observed without diiculty and can even bemeasured. If the viewing beam is bent along the axis by a suitabledeflection field, observation is, in addition, no longer disturbed bythe vapor emanating from the object struck by the beam, i.e., perfectviewing over a prolonged period is guaranteed even when a high-powerbeam is used.

In some cases, particularly for checking large impact spots, it may beof advantage to image the impact spot by means of the particle and/orX-rays reflected from it through a small pinhole diaphragm onto aradiation detector providing information on the size and shape of theimage formed. While a device for applying this viewing method is ofextremely simple design, the check it makes possible will, in manycases, be entirely sufficient.

In a particularly simple application of the new technique, the particleand/or X-ray radiation reflected from the beams impact spot may also beused to project the shadow image of a knife edge onto a radiationdetector. ln this case, the radiation detector must above all provideinformation on the edge sharpness of the image formed. When theseparation between impact spot, knife edge and radiation detector isknown, the size of the impact spot can be directly deduced from the edgesharpness obtained. If a closed edge, i.e` a hole, is used, the courseofthe edge sharpness of the shadow image will, in addition, provideinformation on the shape of the beams impact spot.

It is obvious that in the latter case the diameter of the hole may beconsiderably larger than if the impact spot were imaged through apinhole diaphragm. Another advantage of the new method is the fact thatit also permits the surroundings ofthe beams impact spot to be observedin a simple manner. ln this case, the surroundings to be observed arecovered with an additional, preferably defocused beam. Together with theimpact spot of the beam employed for machining, the area covered isimaged on a radiation detector, preferably a fluorescent screen, by theparticle and/or X-ray radiation reflected from the area covered by theadditional beam. The result is a bright image of the impact spot whichpermits exact checking of the size and shape of this spot, and a darkerimage of the surroundings which permits the beam to be exactlypositioned.

If a beam of sufficiently high aperture is used for machining, thefocusing arrangement can be chosen so that only part of the beam isemployed for the machining process, while another part of the beamcovers the surroundings to be imaged. In this case, a specialilluminating beam is thus no longer required. Taking into account thespherical aberration of the imaging lens, the beam may, for example, befocused so that its central portion is used for machining, while anouter, annular zone of the beam illuminates the surroundings to beobserved.

If the machining process permits short-time interruptions (pulsed beam,intermittent operation), the beam serving for machining may also b eused to form an image of the surrounding area without any additionaloptical means. During the short interruptions in machining, the beam isthen either deflected rapidly, e.g. in the form of scanning, to coverthe area to be imaged, or its impact zone is artificially enlarged bymeans of defocusing.

A preferred device for the application of the method covered by thepresent invention consists of a viewing system which contains at leastone charge-carrier-optical imaging lens and one radiation detector andwhich is arranged so that it intercepts at least part of the corpuscularradiation reflected from the beam s impact spot.

Of particular advantage is the use of a fluorescent screen as aradiation detector, which will then directly display a true-toscaleimage of the impact spot.

Under certain conditions, it may also be advisable to use appropriatedeflection fields in order to project the image by a scanning-typemotion onto a photoelectric detector of lesser size than the image,whose voltage controls the intensity of an oscillograph ray synchronizedwith the scanning motion. Inv

' this case, any desired image scale can be chosen.

If only the size of the beams impact spot is to be checked, theradiation detector may also be a photoelectric detector, which is againsmaller than .the image of the impact spot formed on it at the desiredoptimal focusing of the beam, and which is connected to an indicator.

, When this radiation detector is moved to the center of thecorresponding image, the indicator connected to it will show maximumdeflection when the beam is optimally focused. When the detector ismoved to the edge of the corresponding image, a minimum deflection willbe obtained when the beam is optimally focused.

If a beam of charged particles is used for machining, charged particlesare reflected from the impact spot of the beam, and these chargedparticles moving away from the impact spot at high speed are thenintercepted by the viewing systems. At the same time, X-ray radiation isalso reflected from the impact spot of the beam, which may be used asdescribed above for checking the shape and/or size of the beam s impactspot.

lf a viewing system is used for checking the size and shape of theimpact spot, which images the particle radiation reflected from theimpact spot by charge-carrier-optical means on a radiation detector,preferably a fluorescent screen, this viewing system may be arranged ina tilted position with respect to the axis of the corpuscular beam sothat it intercepts the corpuscles reflected to one side. In thisconnection, it will be found particularly advantageous to bend the axisofthe viewing system, for example by a magnetic deflection field, inorder to protect the fluorescent screen from evaporation, The viewingbeam may be bent at any desired point, i.e. both between the impact spotand the image-forming lens and between the lens and the radiationdetector.

The viewing system can also be arranged at right angles to the axis ofthe beam. In this case, it is indispensable that a deflection field beavailable between the impact spot and the viewing system for deflectingthe particles reflected sideways towards the viewing system. Noadditional means are therefore required to protect the fluorescentscreen from evaporation.

The viewing system may contain one or two image-forming lenses. It isparticularly,advantageous to provide two imageforming lenses and to usethe one serving to focus the beam as which maintains the current flowingthrough the focusing lens Aa first image-forming lens. In this case,appropriate means, preferably magnetic deflection fields, have to beprovided for separating the beam used for machining from the reflectedparticle radiation.

This arrangement offers another special advantage if it is required forthe particular machining application to guide the machining beam, e.g.with the aid ofa deflection system below the lens, in the desired mannerover the workpiece. If in this case above all the elastically reflectedcharge carriers are used for imaging the impact spot, and if anelectrical deflection field is employed for deflection, the image of theimpact spot will remain stationary on the fluorescent screen even whenthe beam moves over the surface of the workpiece.

lf a low-power beam is used for machining or if the viewing systemimages the impact spot of the beam at a particularly high magnification,the image formed on the fluorescent screen may be too weak. ln thiscase, the viewing system should preferably be equipped with an imageconverter tube. Another possibility is to increase the energy of thereflected particle radiation by post-acceleration, in order to obtain abrighter `screen image.

The device covered by thc present invention can also be used toautomatically adjust a focus of minimum size. ln this case, anarrangement may, for example, be provided which periodically changes thecurrent flowing through the lens serving to focus the beam. Aphotoelectric detector is then arranged in front of the fluorescentscreen of the viewing system, which will, for example, cover only thecentral portion at a constant level when the detector current hasreached a peak. With the aid of this device, the current flowing throughthe focusing lens is thus automatically adjustedso that a focus ofyminimum size will be produced. This automatic adjustment of aminimum-size focus can also be obtained if only a photoelectric detectoris used as a radiation detector instead of a fluorescent screen.

Having briefly described the invention, it will be described in gre aterdetail, along with other objects and advantages thereof, in thefollowing detailed description which may be more easily understood byreference to the accompanying drawings, of which:

FIG, l is a partly sectioned side elevation of a device in accordancewith the present invention;

FIG. 2 is an enlarged view of a portion of the device shown in FIG. l;

FIG. 3 is a cross-sectional view of another embodiment of a viewingsystem;

FIG. 4 is a perspective view of a viewing system using an oscillographtube for image formation;

FIG. 5 is a elevation view of a viewing system equipped with apost-acceleration section;

FIG. 6 is a partially sectioned elevation view of embodiment of thepresent invention in which the lens serving to focus the beam used formachining is employed as the first image-forming lens of the viewingsystem;

FIG. 7 is a partially sectioned elevation view of another embodiment ofa viewing system according to the present invention in which thefocusing lens is likewise used as the first image-forming lens;

FIG. 8 is a partially sectioned elevation view of another embodiment ofthe present invention in which the X-ray radiation reflected from theimpact spot of the beam is imaged with the aid of a pinhole diaphragm;

FIG. 9 is a partially sectioned elevation view of another embodiment ofthe present invention in which the radiation reflected from the impactspot projects an image of a knife edge onto a fluorescent screen;

FIG. l0 is a perspective view of another embodiment of the presentinvention in which the radiation reflected from the impact spot images aknife edge on a fluorescent screen;

' FIG. 1l is an elevation view of an embodiment of the present inventionin which the radiation reflected from the impact spot projects the imageof a pinhole diaphragm onto a fluorescent screen;

FIG. 12 is a partially sectioned view of an embodiment of the presentinvention which is suitable for the automatic adjustment of a focus ofminimum size.

In FIG. 1, there is shown a device l for machining material by means ofan electron beam. The beam-generating system of this device consists ofthe cathode 2, the control electrode 3 and the grounded anode 4. Theunits 5 and 6 serve to generate the heating voltage for the cathode 2,the bias voltage for the control electrode 3, and the high voltage. Inthe direction of the beam, below the anode'4, is an electromagneticdeflection system 7 serving for beam adjustment. The unit 8 is the powersupply unit of the deflection system 7. Below the system 7 is adiaphragm 9 which can be displaced in the diaphragm plane in a mannernot illustrated in the drawing.

The electromagnetic lens l0 serves to focus the electron beam 12 on theworkpiece 13. The lens 10 is supplied with power by the unit 11. Theworkpiece 13 rests on the schematically represented universal stage 14which can be shifted from left to right by means of the crank 15 andfrom front to rear by another crank not shown in the drawing.

A portion 16 of the electrons reflected from the impact spot of theelectron beam 12 on the workpiece 13 reaches the viewing system 20. Thisconsists of the two image-forming lenses 17 and 18 which project animage of the impact spot of the beam onto the fluorescent screen 19. Thesize and shape of the beams impact spot can be directly determined fromthe magnified image of the impact spot on the fluorescent screen 19. Athin diaphragm 50 is arranged in the plane of the first aerial image,which ensures that only electrons of a-certain predetermined speed reachthe fluorescent screen 19; The viewing system 20 is bent behind the lens18, and a deflection field 21 (schematically illustrated) deflects theelectrons 16 towards the fluorescent screen 19. The deflected path ofrays in the viewing system protects the fluorescent screen 19 fromdamage and coating by` material evaporated from the workpiece duringworking.

lt is also possible to replace the fluorescent screen 19 by aphotoelectric detector connected to an indicator. This detector must beslightly smaller than the image of the beams impact spot formed on thedetector when the desired optimal focusing of the electron beam 12 hasbeen achieved. With this arrangement the focusing state of the electronbeam 12 can be directly indicated by the deflection of the indicatorspointer.

FIG. 2 is an enlarged view of a portion of the device shown in FIG.- 1.A high-aperture beam is illustrated. Utilizing the spherical aberrationof the focusing lens l0, the focusing of the beam 22 is chosen so thatonly the shaded central portion of the beam 22 is focused on theworkpiece 13. The annular zone of the beam surrounding this centralportion is here used to illuminate the surroundings of the beams impactspot. With the aid of the viewing system 20, a bright image of theimpact spot and a darker image of the surroundings of this spot can thenbe observed.

lt is also possible to use the device shown in FIG. l in order at thesame time to observe the surroundings of the beam s impact spot. Forthis purpose, the power supply unit 11 of the lens need only becontrolled so that` the lens alternates between focusing and defocusingthe beam 12. Furthermore, a deflection system can be arranged below thelens 10 as is shown, for example, in FIG. 6, and a voltage can be fed tothis deflection system, which will cause the beam l2 to stop in therespective machining position during successive intervals and to bedeflected during the intermediate intervals so that it will cover thesurroundings of the working point in a scanning-type pattern.

In the embodiment illustrated in FIG. 3, the viewing system consists ofan image-forming lens 23 and the fluorescent screen 24. The viewingsystem is here arranged at right angles to the axis of the electronkbeam 12. Ari electromagnetic deflection field 25 serves to deflect theelectrons reflected from the impact point of the beam into the axis ofthe viewing system 23, 24. This arrangement offers the advantage thateven without a thin diaphragm it is essentially only electrons of apredetermined speed which reach the fluorescent screen 24. In addition,this arrangement may be made somewhat more compactly than that of FIG.1, and the fluorescent screen is automatically protected fromevaporation.

Instead of a viewing system provided with a fluorescent screen, a deviceof the type illustrated in FIG. 4 may be used. Here, the particleradiation passing through the image-forming lens 23 is movedby adeflection field applied between the deflector plates 26, 27 and 28, 29in a scanning-'type pattern over the photoelectric detector 30 which issmaller than the image of the impact spot. The voltage generated by thedetector is amplified by the amplifier 3l and controls the intensity ofthe oscillograph ray via the control electrode 33 of the oscillographtube. The tube 32 contains a deflection system consisting of the platepairs 26', 27' and 28', 29', to which deflecting voltages are applied insynchronism with those applied to the plates 26, 27 and 28, 29. Sincethe dellectng voltages fed to the systems 26, 27 and 28, 29 as well as26', 27', 28', 29' are, moreover, proportional to each other, an imageof the impact spot of the beam l2 is thus formed on the fluorescentscreen 34 ofthe tube 32.

FIG. 5 shows a viewing system which is particularly suited to thoseapplications in which the energy of the particle radiation reflectedfrom the workpiece is not sufficient'for forming a desirably brightscreen image. Here, a post-acceleration section is inserted between theimage-forming lens 23 and the fluorescent screen 38, where the twoaccelerating electrodes 35 and 36 are provided.. The fluorescent screen37 is in the conventionalmanner provided with a thin metal foil 37 whichvis connected to the accelerating electrode 36. The generator 39 servesto generate the post-acceleration voltage.

In the example shown in FIG. 6, the beam-generating system 2, 3, 4 isdisposed at an angle to the axis of the focusing lens l0. Anelectromagnetic deflection field 40 serves to deflect the electron beaml2 into the axis of the lens 10. The latter focuses the electron beam l2in the conventional manner on the workpiece 13. The electrons 16reflected from the impact spot of the beam on the workpiece 13 passthrough the focusing lens 10 and are deflected into the axis of theimage-forming lens 4l by the deflection field 40. In this case, theviewing system consists of the first image-forming lens, which is herethe focusing lens 10, the second image-forming lens 41 and thefluorescent screen 42. The focusing lens 10 forms an image of the beamsimpact spot at 43, looking directly down on the spot, a desiredarrangement in many applications. The deflection field 40 may also bedisposed between the lens 10 and the workpiece 13.

Since in the aforementioned example the focusing of the electron beam 12by means of the lens 10 influences also the focusing in the image plane43, the image plane shift must be compensated for by the lens 41 inorder to ensure that the image of the focus is always sharply defined onthe fluorescent screen 42. For this purpose the power supply units 44for the imaging lens 41 and 45 for the focusing lens 10 areinterconnected.

lf below the focusing lens 10 an electrostatic deflection system 46 isused to displace the electron beam 12 on the workpiece 13, the image ofthe focus formed on the fluorescent screen 42 will remain stationaryeven when the electron beammoves along the workpiece 13.

The arrangement shown in FIG. 6 serves at the same time as an ion trap,thus increasing the life of the cathode. In addition, the fluorcscentscreen 42 is automatically vprotected from evaporation of material fromthe workpiece.

In the example illustrated in FIG. 7, reflected particles 74 passinglaterally of the main beam 12 through the lens l0 are separated by adeflection field 47 and projected onto the fluorescent screen 49 bymeans of an image-forming lens 48. ln this case also, the focusing lens10 is used as the first imaging lens of the viewing system, and thefluorescent screen is automatically protected from evaporation.

In the example shown in FIG. 8, the particle and/or X-ray radiation 50reflected from the impact spot of the electron beam 12 on the workpiece13 is imaged on a fluorescent screen 52 via a small pinhole diaphragm51. This embodiment is particularly useful when large foci have to beimaged. Itis advisable to provide a deflection field in the path of theviewing beam in order to protect the fluorescent screen.

In FIG. 9, a knife edge 53 is arranged on one side of the workpiece 13.This knife edge is imaged on the fluorescent screen 55 by the particleradiation reflected from the impact spot 54 of the beam in the form of ashadow. Between the knife edge 53 and the fluorescent screen 5S adeflection field 56 is indicated schematically, which serves to deflectthe particle radiation. This deflection field and the protective shields57 and 58 arranged in front of the fluorescent screen 55 prevent theaccumulation of evaporated material on the fluorescent screen.

The deflection field may also be arranged between the workpiece 13 andthe knife edge 53 to protect the knife edge 53 from evaporation as wellas the screen. f

The size of the beams impact spot 54 can without difficulty be deducedfrom the sharpness range of the shadow image formed on the fluorescentscreen 55. Thus, an optimally focused beam will produce a sharptransition from bright to dark on the fluorescent screen, while thistransition is gradually weakened as the focus changes from the optimum.

If the knife edge 53 is replaced by a circular diaphragm, a shadow imageof this diaphragm will be formed on the lthis moment 7'y fluorescentscreen' 55. The transition from bright to dark will again' be sharplydefined when the beam 12` isoptimally focused. When the impact point 54is enlarged, the transition from bright to dark will be uniformlyweakened along the entire periphery of the shadow image only if theimpact spot is circular in shape. Any deviation from circular shape isrevealed byvdiffei'enty width of the transition zone from bright todark, so that in this case it is also possible to obtain information onthe shape of the impact spot. j

The deflection field 56 shown in FIG. 9 may, under certaincircumstances, disturb the projection of the shadow image. FIG.therefore shows a device in which such a disturbance is largely avoided.The particle radiation 59 reflected from the beams impact spot 54 isdeflected by the magnetic field formed between the pole shoes 60 and 61and; projects *a shadow image of the knife edge 62 onto the fluorescentscreen 63. In this instance, the magnetic field acts in the direction ofthe knife edge 62, so that the radiation isfdefl'ected around ahorizontal axis. The deflected radiation follows an oblique path to therear, while the direction of the magnetic field is obliquely forward. e

In the device presented schematically in FIG. 11, the radiationreflected from the workpiece 13 passes through a'n annular diaphragm 64which can be displaced in the direction 'of the` arrow. A stop65 servesto cap the center of the diaphragm 64.1When the beam 12 has beenoptimallyfocused-a's is iri-v dicated by the solid lines-the diaphragms64 and 65 can be so ladjusted vone with respect to the other that thefluorescent screen 66 remains dark. When the beam 12 is thendefocused-as is indicated by the dashed lines-a brightannulus willappear on the fluorescent screen 66, the total brightness of whichdepends on the focusing state.

Behindthe fluorescent screen66 isa photoelectric detector 67. lfthisdetector is connected-to an indicator, it is possible to` ,deduceoptimal focusing of the beam 12 from minimum `deflection of theindicator. The detector 67 can also be used in image where brightnesswill be smallest at optimum focusing ofthe beam. This detector isconnected to an amplifier 71 which in turn is connected to a unit 72.This unit generates a switching pulsewhen the voltage supplied bythedetector 70 has reached a minimum value. The unit 73 `influences, thepower supply unit 11 of the'lens insuch manner that the curfent flowingthrough the focusing lens l0 is periodically varied. This causes alsothe sizeof the beams impact'spot on the workpiece 13 to be variedperiodically. During this varia-l tion, the focus passes through thestate of optimal focusing,

and at this instant a switching pulse is generated bythe unitf72 whichcuts off the unit 73 and thus keeps the current, vwhich at flowsthroughl the focusing lens, at a uniform level. i A focus of minimumsizeis thus adjusted automatically.

Instead of the viewing system 68 with the lfluorescent screen i 69 itisalso possible yto `u se a viewing system which is directly Vbased-cnaphotoelectric vdetector instead of theffluorescent screen'e69.

,What is claimed is: l v .Y l. A device for monitoring the focused stateof a beam of charged particles at 'the impact spot ori aworkpece,.compris ingmeans for detecting energy radiated from the impactspot of the beam of charged particles, and a `diaphragm having a sharplydefined edge positioned between the impact spot and the detecting meansto cast a shadow on the detecting means with the sharpness of the shadowrepresentative of the focused state of the beam at the impact spot.

2. The device as recited in claim 1 wherein the diaphragm includes aknife edge to cast a linear shadow.

3. The device as recitedin claim 1 wherein the diaphragm is formed of afirst apertured plate and a second disc spaced w from the plate along apath between the impact spot and the detecting means, with the discselectively size'd with respect to the aperturefin the plate to lpreventradiation from the impact spot from reaching the detection meanswhenever the size of the impact spot is smaller thana predeterminedvalue.

- 4. The device as recited in claim 3 wherein the aperture and the discare' circular.

5.v The' device as recited lin kclaim 3 'wherein distance between thedisc and the aperture is adjustable. v

6. The device as recitedin claim 3 wherein the detecting means comprisesa fluorescent screen and a photoelectric detector arranged adjacent thescreen to sense the light emission l thereof in the area where radiationis received from the impact spot whenever the size of thei'ii'ipact spotis'larger than said predetermined value; f. i

7. The device as recited in claim 3 and further including means forperiodically varying the focusing condition of the beam, means forproducing an voutput signal from the detection means in dependence onthe radiation received thereon, ai'idmeans for stopping the variation ofthe focusing condition upon occurrence of a minimum of the outputsignal. l

8. The device as recited in claim l wherein the detecting means`comprises a iluorescentscre'en. 9. The device as recited in claim 1 andlfuitherincluding means for deilectingcharged particle'radiation fromthey i m pact spot onto the detecting means.

10. The device as recited in claim l whereinV the detection means areresponsive to charged particle radiation.

l1.-A method ofmonitoring the focused state of a beam of chargedparticles at its impact spot on a workpiece, comprising the steps ofpositioning an impact spot radiation sensitive detection devic'ehat alocation to receive energy vradiated from spot, and sensingthebrightening of the'. dark shadowk image upon a defocusing of the beam atthe impact spot and producing a signal representative thereof.

1. A device for monitoring the focused state of a beam of chargedparticles at the impact spot on a workpiece, comprising means fordetecting energy radiated from the impact spot of the beam of chargedparticles, and a diaphragm having a sharply defined edge positionedbetween the impact spot and the detecting means to cast a shadow on thedetecting means with the sharpness of the shadow representative of thefocused state of the beam at the impact spot.
 2. The device as recitedin claim 1 wherein the diaphragm includes a knife edge to cast a linearshadow.
 3. The device as recited in claim 1 wherein the diaphragm isformed of a first apertured plate and a second disc spaced from theplate along a path between the impact spot and the detecting means, withthe disc selectively sized with respect to the aperture in the plate toprevent radiation from the impact spot from reaching the detection meanswhenever the size of the impact spot is smaller than a predeterminedvalue.
 4. The device as recited in claim 3 wherein the aperture and thedisc are circular.
 5. The device as recited in claim 3 wherein distancebetween the disc and the aperture is adjustable.
 6. The device asrecited in claim 3 wherein the detecting means comprises a fluorescentscreen and a photoelectric detector arranged adjacent the screen tosense the light emission thereof in the area where radiation is receivedfrom the impact spot whenever the size of the impact spot is larger thansaid predetermined value.
 7. The device as recited in claim 3 andfurther including means for periodically varying the focusing conditionof the beam, means for producing an output signal from the detectionmeans in dependence on the radiation received thereon, and means forstopping the variation of the focusing condition upon occurrence of aminimum of the output signal.
 8. The device as recited in claim 1wherein the detecting means comprises a fluorescent screen.
 9. Thedevice as recited in claim 1 and further including means for deflectingcharged particle radiation from the impact spot onto the detectingmeans.
 10. The device as recited in claim 1 wherein the detection meansare responsive to charged particle radiation.
 11. A method of monitoringthe focused state of a beam of charged particles at its impact spot on aworkpiece, cOmprising the steps of positioning an impact spot radiationsensitive detection device at a location to receive energy radiated fromthe impact spot, selectively intercepting the flow of said energy tosaid detection device with a sharply defined edge to cast a shadow imageon the radiation detection device with the sharpness of the shadow beingrepresentative of the focusing condition of the beam at the impact spot.12. The method as recited in claim 11 and further including the steps ofadjusting the interception of the radiation to cast a normally darkshadow image for a focused beam at the impact spot, and sensing thebrightening of the dark shadow image upon a defocusing of the beam atthe impact spot and producing a signal representative thereof.